The present invention relates to evaporators and in particular to so-called flash evaporators to be utilized for purposes such as distilling sea water.
The present invention relates especially to evaporators which operate with relatively high liquid flow rates at relatively low temperatures.
In conventional flash evaporators utilized for purposes such as distilling sea water, the liquid which is to be distilled flows from one distilling stage to the next with the successive stages operating at progressively lower pressures, and the liquid in each stage flows through an orifice so that part of the liquid evaporates, drawing the energy required for this purpose from the remaining liquid which thus is correspondingly cooled.
The operation of installations of this type require that the temperature of the fluid should have an equilibrium with the prevailing vapor temperature, so that the energy content of the liquid can be utilized as fully as possible. In practice, however, such equilibrium is not fully achieved. This equilibrium state is dependent upon a number of factors including the prevailing temperature, the differential temperature from one stage to the next, the design of the orifice through which the liquid flows in each stage, the length of each stage, and the quantity of liquid flowing therethrough. This latter factor, namely the quantity of liquid flowing through each stage, plays a highly significant role particularly at low temperatures and when the interstage differential pressure is low. In addition, this factor of the quantity of liquid flowing through each stage becomes very important when the liquids which are distilled have a high viscosity, as, for example, when industrial waste waters are being concentrated. Under the latter circumstances, a major liquid flow requires the discharge of a thick liquid layer through the orifice in each distilling stage. Inasmuch as the vapor pressure of the liquid is only slightly higher than the prevailing pressure, only the uppermost layer of the liquid is capable of evaporating satisfactorily, while boiling of the liquid at a greater depth is prevented by reason of the hydrostatic head of the liquid. As a result, the above equilibrium conditions cannot be achieved, and part of the liquid passes through each distilling state at an excessively high temperature.
In attempting to distill water by utilizing the waste heat contained therein while at the same time providing extremely low pressures, teachings as disclosed in U.S. Pat. Nos. 3,630,854 or 3,783,108 may be utilized for achieving the low pressure by situating the evaporator at a barometric height over the free water surface. However, if a conventional evaporator structure is used under such conditions, with the condenser situated at an upper part of the evaporator, the cooling water required for the condenser must be pumped up to an excessive height, losing in this way a certain amount of pumping energy and creating difficulties with respect to air dissolved in the cooling water and separating therefrom in the condenser tubes.
Futhermore, at relatively low temperatures, the vapor has a very high specific volume. This factor creates the need of wide vapor passages in the evaporator or high vapor velocities. When high vapor velocities are utilized there is a well known risk that droplets of liquid are entrained with the vapor and will therefore deteriorate the quality of the distillate. In order to avoid this problem it has been common to use various types of screen or net-like droplet separators. When operating at low temperatures with exceedingly low differential temperatures between the successive stages, such droplet separators create undesirably high pressure losses which in turn increase the required heat-exchange surface area and the cost of the installation.